The invention relates to a method for producing a discharge lamp having a discharge vessel filled with a discharge medium and having an electrode running at a distance from the vessel faces in the discharge vessel, wherein during the production of the discharge vessel, an exhaust tube through which the discharge vessel is evacuated and/or filled with the discharge medium, and which is subsequently sealed, is applied to the discharge vessel.
A small tube or the like, which projects through a wall of the discharge vessel, is customarily co-fused as an exhaust tube during the production of the discharge vessel. After the discharge vessel has then been sealed hermetically, apart from the exhaust tube, an evacuating pump is externally connected to the exhaust tube and the discharge vessel is evacuated to the desired reduced pressure. The discharge vessel is subsequently filled through the exhaust tube with the discharge medium, in general a gas or gas mixture. After the gas conditions in the discharge vessel have been adjusted suitably, the small tube forming the exhaust tube is fused off outside the discharge vessel, in general relatively close to the discharge vessel, in order to seal the discharge vessel in a completely vacuum-tight fashion. The exhaust tube is customarily applied to the discharge vessel in such a way that it does not project into the interior of the discharge vessel.
Discharge lamps with dielectric barrier discharge are a special type of discharge lamp. In this case, the electrode and/or a counter electrode or a plurality of counter electrodes are separated, by at least one dielectric, from the discharge in the interior of the discharge vessel. The invention pertains not exclusively, but preferably to such discharge lamps for dielectric barrier discharges.
In such discharge lamps, a voltage is applied between the electrode and a counter electrode, which is likewise located in the discharge vessel or on the discharge vessel, with the result that a discharge takes place, between the electrode and the counter electrode, in the discharge medium. Incoherent radiation is released during this discharge. Depending on the type of lamp design, and especially on the discharge medium, ultraviolet or infrared radiation or visible light is radiated. Such a lamp is described, for example, in DE 196 36 965 A1.
Distinction is made between discharge lamps operated in the so-called bipolar mode and in the so-called unipolar mode.
In the case of a unipolar discharge lamp, one of the two electrodes permanently forms the anode, and the other electrode forms the cathode. It is then sufficient to separate the anode from the discharge by the dielectric. The vessel wall can also be used as the dielectric for this. In this case, only the cathode is located in the discharge vessel, while the anode or anodes are externally applied to the vessel wall. These may involve, for example, strips of a gold or platinum paste, which are printed directly onto the vessel wall.
In the case of the bipolar discharge lamp, the two electrodes are not distinct from one another, and both are screened by a dielectric. By corresponding voltage alternation, first one electrode and then the other electrode forms the anode. Preferably, the discharge lamps are supplied with an in principle unrestricted sequence of extremely short voltage pulses at high repeat frequency. In this context, reference is made to WO94/23442.
It is clear that a successful and efficient discharge depends on the distance from the electrode to the counter electrodes. This is particularly relevant in the case of discharge lamps with a dielectric barrier discharge, for which the efficiency depends strongly on the accurate geometry of the discharge regions in the discharge vessel. Because of this dependency on the electrode geometry, even minor changes in the position of an electrode would lead to a large efficiency reduction of the discharge lamp.
It is therefore an object of the present invention to provide a cost-efficient and simple method for producing a discharge lamp, in which accurate seating of the electrode during production and in subsequent use is guaranteed.
This object is achieved in that a frame made of non-conductive material, which fixes the electrode at a predetermined position in the discharge vessel, is arranged in the discharge vessel, and this frame or a part of this frame is designed and used as an exhaust tube or part of an exhaust tube.
According to the invention, the exhaust tube which is in any case needed for the production of a discharge lamp is simultaneously used to act as a frame or part of a frame. Reliable positioning of the electrode during production and in subsequent use is thereby possible, and additional method steps can be minimized or additional method steps can even be avoided. In other words, the cost of production is not increased by the method according to the invention, or is only increased insignificantly.
The frame, or the part of the frame used as an exhaust tube, can in principle have any desired shape that is suitable for fixing or supporting the electrode correspondingly. For use as an exhaust tube, it is merely necessary for the corresponding part of the frame to have an opening passing from the outside inward into the discharge vessel, and a possibility for hermetic connection of the evacuating and filling system.
In a preferred embodiment of the method according to the invention, the part of the frame used as an exhaust tube is designed in the form of a tube, which projects with one end through a wall of the discharge vessel into the discharge vessel. For that purpose, it is basically only necessary to lengthen the customary exhaust tube used hitherto.
This method is advantageous especially when the electrode has an elongate shape extending substantially in a main extension direction. The frame can be designed so that it has an elongate support element which runs in the main extension direction of the electrode and supports the electrode over at least one sub-region, the support element and the electrode being aligned coaxially or parallel with one another.
In this case, the aforementioned inwardly lengthened exhaust tube can be used as a support element or support tube. It is consequently possible, by corresponding arrangement of the tube projecting into the discharge vessel with respect to the electrode, to guarantee simple and reliable support of the electrode.
Of course, the support element may also be formed in a different way, for example an elongate plate-like support element or the like, which is provided with a corresponding tube part or has a corresponding bore. Likewise, the electrode may also have any desired shape. For instance, it may comprise lengthwise strips, a wave shape, for example in a sine shape, zigzagged electrodes, a coil or the like.
In a particularly preferred embodiment, the electrode runs in a coil shape around the support element at least over a sub-region of the lengthwise extent. The advantages of this coil electrode, and of the resulting structure of the discharges, are presented in the previously cited DE 196 36 965 A1.
The electrode can be applied to the frame, or to an element of the frame, in the form of a correspondingly structured conductive coating, for example of gold or platinum paste. For instance, a coil-shaped inner electrode can be applied in this way to a support rod or support tube running in the extension direction of the electrode. Electrodes in the form of strips, waves or zigzags can, for example, be applied to an elongate, plate-shaped support element.
In a particularly preferred embodiment, the discharge vessel is cylindrical with an inner electrode extending along the cylinder axis and with a support element. It is consequently a discharge lamp with a central or concentric middle electrode. It is suitable in this case for a plurality of strip-shaped counter electrodes, running parallel with the cylinder axis, to be applied to the vessel walls. The entire discharge space can be used effectively in this way, the geometrical arrangement needed for this being relatively simple to create. The problem, which then arises, of an exact central or concentric position of the middle electrode during production and constant operation is solved by the invention.
The support element, for example in the form of the support tube used as an exhaust tube, advantageously projects into the discharge vessel from an end side opposite a lamp cap, and is fastened to the corresponding end face. Especially in the case of fairly long discharge vessels, however, capping on both sides may also be advantageous. The exhaust-tube side is then also covered by a cap.
Of course, the support element or the tube may also be long enough for the electrode to be supported over the entire length.
Preferably, one or more openings which pass through the tube wall, and are located inside the discharge vessel, are made in the support tube along the lengthwise extent. Through these openings, better and more effective pumping and filling of the discharge vessel is possible.
In a preferred embodiment, the discharge vessel is sealed on one side by pinching. In this case, a metal foil is co-embedded and acts as an electrical feed-through for the inner electrode. The latter is fastened, for example soldered, to the metal foil and is partly co-pinched with one end, so that the electrode is held on one side by the pinch seal.
In order to prevent displacement of the electrode in the lengthwise direction of the support element, the latter advantageously has a retaining projection extending transversely with respect to the lengthwise direction of the support element, or with a retaining recess. The retaining projection may comprise corresponding bumps, lugs, spikes etc., and the retaining recess may be a groove or the like. Using such bumps or lugs, a wire coil which runs around a support tube or a support rod may, for example, be secured very well against lengthwise displacement.
In an alternative embodiment of the frame, a spacer is located between the electrode and a vessel wall, transversely with respect to the main extension direction of the electrode. Such a spacer may be a hook-like element, a small tube or a pin which is fastened to a vessel wall and holds the electrode in position. A plurality of such spacers are preferably used along the main extension direction of the electrode. In order to permit the function as an exhaust tube, of course, a corresponding bore must be made in at least one of the spacers.
Of course, it is also possible to use a, for example, tubular support element and additionally spacers running transversely with respect thereto. This is suitable, in particular, for the production of very long lamps.
In a preferred embodiment, this additional spacer for fixing the support element is a retaining disk, which is arranged transversely with respect to the main extension direction of the electrode, which extends between the vessel faces, and through which the electrode with the insertion element runs at a predetermined position. Such a retaining disk guarantees reliable positioning in every direction.